Method and apparatus for spraying powdered thermoplastic substances

ABSTRACT

POWDERED THEMOPLASTIC SUBSTANCES ARE SPRAYED IN HEATED FORM AGAINST A SUBSTRATE, BY ENTRAINING THE POWDER COLD IN A GAS STREAM, AND EJECTING THE GAS STREAM FROM A NOZZLE IN THE FORM OF A FLAT CLOUD. THE CLOUD IS CONFINED BETWEEN A PAIR OF FURTHER GAS STREAMS AND IS HEATED ON THE WAY TO THE SUBSTRATE. HEATING MAY BE EFFECTED BY PASSING THE CLOUD SANDWICHED BETWEEN ITS CONFINING STREAMS, BETWEEN A PAIR OF OPPOSED ELECTRIC HEATING ELEMENTS. ALTERNATIVELY, HEATING MAY BE EFFECTED BY HEATING THE CONFINING STREAMS THEMSELVES, PARTICULARLY IF THE CONFINING STREAMS ARE FLAMES EMERGING FROM A BURNER.

Feb. 6, 1973 r. KENDERI 3,11%076 METHOD AND APPARATUS FOR SPRAYINGPOWDERED THERMOPLASTIC SUBSTANCES Filed April 23, 1971 In venlor 7700,?Aim/05x? 35% raw A Horn 9 y:

United States Patent 3,715,076 METHOD AND APPARATUS FOR SPRAYIN GPOWDERED THERMOPLASTIC SUBSTANQES Tibor Kenderi, Budapest, Hungary,assignor to Hajtomu es Felvonogyar, Budapest, Hungary Filed Apr. 23,1971, Ser. No. 136,936 Int. Cl. 1305b 1/24, 1/28; C23c 7/00 US. Cl.239-8 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to amethod and apparatus for the spraying of powdered substances,particularly of thermoplastic powdered substances.

' Numerous methods and devices are conventional for the spraying ofthermoplastic powdered substances, all of which serve for the productionof coatings on various surfaces. The gravest and most importantdisadvantage exhibited by the conventional devices resides in the factthat the uniform and reliable heating of the appropriate amount of spraypowder, as well as the uinform metering thereof in the desired quantity,especially after there has been an interruption and resumption of thespraying procedure, are not ensured. Furthermore, the spray nozzles ofthe devices known heretofore do not ensure a uniform thickness of thecoating, and the width of the spray jet cannot be varied withoutexpanding additional energy. Additionally, there is the disadvantage inthe conventional devices that they are unsuitable, without some changein the equipment, for the intermittent or continuous spraying not onlyof powdered substances exhibiting a high melting point but also of thoseexhibiting a low melting point. A further disadvantage resides in thefact that, in these devices, the transition from one type of powder toanother, and the cleansing of the device, can be conducted only withdifiiculty and only after re-assembling the device.

In the conventional spraying methods and apparatus, the metered powderis sprayed in the shape of a cone; and consequently, it is impossible toensure the uniform heating of the powder or the uniform layer thicknessof the coating. The devices provided with such nozzles, althoughsuitable for purposes of backfill welding, are unsuitable for theproduction of a corrosion-resistant coating requiring a uniform layerthickness over a large surface area.

A powder spray device is also known wherein the mixing of oxygen or airand fuel gas is effected by means of nozzle bores arranged inparallelism to each other in two concentric circles. In this device, aguide ring connected to the nozzle by a metal bond is employed forvarying the flame shape. Due to this metallic connection, the nozzle isstrongly heated during operation. In this device, the powder spraynozzle is mounted by means of a fixed and airtight connection providedon the center line of the nozzle. In other words, the nozzle issurrounded on all sides by the flame during operation. Also in thisapparatus, a cone-shaped spray jet results which, accordingly, does notensure that the thus-sprayed powder particles are heated uniformly. Theaxis of the bores serving for the exit of the gaseous mixture producingthe flame is disposed parallel to the axis of the power nozzle;consequently, the lateral expansion of the powder jet is unlimited, withthe result that the planar surface to be coated by means of the sprayfrom this nozzle is provided with a non-uniform coating which is thickerin the center.

Still another device is conventional wherein the powder to be sprayed isconducted by the nozzle through a pipe with a heated wall and isplasticized in this manner. In order to prevent flowing of the substanceparticles emanating from the nozzle on the substrate to be coated, themouth opening of the nozzle is provided with a number of bores throughwhich unheated gas is admixed with the hot gas jet conveying the moltenpowder. During operation, the pipe serving for melting the powder musthave a temperature which is substantially above the melting point of thepowder. Thus, for example when spraying aluminum powder, thistemperature is above 900 C. However, at such a high temperature, thegastight seal and the production thereof are plagued with difliculties.Also in this device, the orifice adjoining the end of the heating pipeis fashioned with a circular cross section, or with a different one, forexample a basket-shaped cross section, However, a consequence of thisshape is that the powder which has been molten or brought to a highdegree of plasticity is subjected, after flowing through the roundheating pipe, to a forcible change in direction in the basket-shapedmouth orifice; and as a result, the powdered substance adheres to thewall of the mouth orifice.

An object of the present invention is to provide a process andconstruction of a powder spraying apparatus universally suitable forpowder types having a high melting point as well as those having a lowmelting point, without exhibiting the disadvantages of theabove-described devices, and ensuring a uniform and reliable heating ofthe powder, as well as, after shutoff and resumption of operation, auniform metering of the powder and a uniform thickness of the coating,without the use of additional energy, and capable of being cleanedeasily without disassembly.

The invention resides in that the powdered substance to be sprayed isspread out in a cold condition, with the aid of a cold gaseous stream,in the form of a flat cloud, the thickness of which cloud must be atleast twice the granular size of the powder to be sprayed and at mostone-eighth inch, the application of the powder to the substrate to becoated being effected by means of the cloud laterally supported by agaseous stream and shaped as set out above, with simultaneous melting bythermal or electric energy.

Accordingly, this invention is based on the surprising finding that acloud-like powder stream spread out flat can be heated in quite a shortperiod of time and without the aid of any larger amount of thermalenergy, over its entire extent, i.e., from the exit of the powderparticles from the discharge orifice of the nozzle until their impingement on the substrate to be coated, to the desired extent.

In the process of this invention, in contrast to all processes knownheretofore, the cloud-like powder jet spread out flat is heated over itsentire extent. Heretofore, it was possible to heat sufliciently only theparticles which were present on the exterior of the atomizing cone;whereas the powder particles moving in the center of the atomized conein the largest quantity and at the highest speed could be heated onlywith difficulty and in an unreliable mannner.

The invention provides not only an apparatus suitable for conducting theprocess of this invention, equipped with a powder container with ametering unit, but also a spraying device suitable for spraying themetered powder.

An important feature of the apparatus of this invention resides in thatthe spraying device is provided with a flat or fishtail nozzle, thewidth of which is at least twice the granular size of the powder to besprayed and which nozzle is provided with a flat outlet orifice of alength of at most ten inches. Another important feature is thatgasconducting bores are arranged on both sides of this outlet orificeand form an angle of about 20 with the midplane or plane of symmetry ofthe spraying device. Still another noteworthy feature is that energysupply elements are provided behind the outlet orifice of the nozzle,which elements extend along the two longitudinal sides of the powder jetspread out by means of the nozzle and defined laterally by gas jets.

This construction of the spraying device ensures cooling of the nozzle,since the air sucked in through an opening between the heating head andthe nozzle ensures intense cooling due to the injector effect of thepowder stream emanating from the spray nozzle. At the same time, thisstructural design also results in a quieter flow of the flame jets,because with the sunken nozzle construction the marginal vortex producedby the exit of the powder stream does not reach the flame jets.

Due to the fact, in turn, that the powder stream, spread out by the flatnozzle, is surrounded on both sides by energy supply elements behind thefiat outlet orifice of the spray device, it is ensured that it ispossible to supply the powdered substances with the thermal energy, aswell as with a diflerent type of energy, e.g. electric energy.

According to this invention, the inlet opening of the ejector servingfor conveying the powder is arranged on the side of the ejector suctionchamber. By this design, the disturbances occurring during start-up andshutoff can be eliminated, since the suction chamber of the ejectorcannot become clogged.

Underneath the powder container of the device of this invention, whichcontainer is open at the bottom, a metering unit is provided which iscontrolled independently of the ejector, which unit is positivelymechanically operated and is provided with an infinitely variablecontrol. In this manner, this metering device can be operatedindependently of the ejector at any time with the required amount ofpower, or the operation can be interrupted or started up again. Due tothe fact that this metering unit is provided with an electrically drivenvibrating chute, bridging over of the powdered substance is avoided bytransmitting vibrations to the system.

With the use of the process or apparatus of this invention, the sprayingof the required amount of powder can be conducted with safety; and anadditional advantage is afforded by the fact that all powder particlescan be heated to the same temperature and that the device is universal,i.e. it is suitable for the spraying of powders of a high melting point,as well as those of a low melting point.

A detailed description of the invention is set forth below withreference to the embodiment illustrated in the drawing, wherein:

FIG. 1 diagrammatically shows the arrangement of the metering unit ofthe powder spraying device according to the present invention;

FIG. 2 is a fragmentary sectional view of the ejector of the device;

FIG. 3 diagrammatically shows the arrangement of the nozzle of thespraying device according to this invention;

FIG. 4 is a horizontal sectional view of FIG. 3;

FIG. 5 is a view of the nozzle of this invention as a heater outletorifice according to a modification of the apparatus; and

FIG. 6 is a horizontal sectional view of the embodiment illustrated inFIG. 5.

Referring now to the drawing in greater detail, there is shown acontainer 1 for the storage of the powder to be sprayed. Container 1 isopen so that the powder can be replenished continuously and in a simplemanner. The

container 1 is fashioned, in the illustrated embodiment, in the form ofa funnel which tapers downwardly and is open at the bottom. The powderedsubstance to be meterd flows through the lower end of this hopper to thevibrating chute 2. The vibrating chute 2 is actuated by anelectromagnet. The feed of the powder present in container 1 can beinfinitely varied by means of a rheostat (not shown) independently ofthe quantity of powder present. By interrupting the current source forthe magnet, the powder feed can be interrupted or started up again atany desired instant, or it can be continued for any desired length oftime. Underneath the discharge end of the vibrating chute 2, a funnel 3is provided. On the side of this funnel, an opening 4 is provided whichis in communication with the ejector 5, which latter is constantlyoperated. The powder dropping from the vibrating chute 2 thus passesthrough the funnel 3 and the opening 4 thereof into the ejector 5, whichlatter then entrains the powder and feeds the same to the nozzle via theconduit 6. By providing the opening 4 on the side of the funnel 3,instead of the structure generally conventional heretofore, the dangeris eliminated that the suction chamber of the ejector 5 will fill withpowder when the device is shut off or when an error occurs duringoperation and thus results in an interruption of the operation of thedevice.

The spraying head essentially consists of the flat fishtail nozzle 7 andthe heater 8, wherein the nozzle 7 is disposed in the longitudinal gapof the heater 8. The powder conveyed through the conduit 6 istransformed, by the nozzle 7, into a finely distributed, cloud-likestream 9 and blown through the heater 8 between the two rows of flames10 arranged on the right-hand and left-hand sides of the burner '8 asseen in FIG. 4. The oxygen or air required for combustion, as well asthe fuel-gas mixture are fed to the burner 8 via the conduit l l.

In order to ensure the interchangeability of the nozzle 7, the latter isindependent of the heater 8 and is connected to the heater in a suitablemanner by heat insulation. In place of heat insulation, it is alsopossible to provide cooling at this point.

The outlet opening of the nozzle 7 is not disposed flush with the frontof the heater "8 but rather is somewhat farther to the rear at adistance therefrom. In the thusdesigned spray head, due to the injectoreffect of the gaseous stream exiting through nozzle 7, air is sucked inthrough the gap between the heater 8 and the nozzle and thereby asignificant cooling effect is obtained. Due to this construction of thespray head, the flow of the flame jets of the row 10 of flames is alsoquieter and more uniform since this flow is not aifected by the marginalvortex of the emerging powder jet.

The nozzle 7 is flat, and the width of its orifice is at least twice thediameter of the powder grains to be sprayed, whereas this width must beno larger than It has been found in practice that the length of themouth orifice of the nozzle 7 must not exceed ten inches.

The outlets of the row 10 of flames on the right-hand and left-handsides of the burner '8 are disposed at a maximum spacing of 0.4 inch asmeasured from the edge of the nozzle 7 in the lateral direction. Thecenter. line of each outlet of the row 10 of flames forms an angle of atleast 5 and at most 20 with the flow direction of the powder jet 9. I

The outlet orifice of the nozzle 7 is spaced about 0.040.3 inch behindthe front of the burner 8.

In operation, the powder container 1 is filled with the powder to besprayed; thereafter, with the aid of the rheostat (not shown), theamount to be fed is set, and in correspondence therewith, the vibratingchute Zexecutes a shaking motion and thereby conveys the powderedsubstance from the container 1 along the vibrating chute 2 to the funnel3 of the ejector 5. The ejector 5 is in continuous operation, whereasthe metering of the powder can be interrupted and/or started up again atany desired instant by switching the magnet of the vibrating chute 2 onor 011. Since, in place of the arrangement generally employedheretofore, the opening 4 on the side of the funnel is arrangedlaterally in the ejector 5, it is impossible for clogging to occur byshutting ofi or erroneous operation of the device, for the suctionchamber thus cannot be clogged.

Through the ejector 5, the powdered substance passes via conduit 6 intothe nozzle 7; the latter then conveys the powder through its flatopening in the form of a cloud-like, finely distributed powder jet 9,into the space between the rows 10 of flames of the heater 8. Duringthis process, the powder jet 9 is sandwiched between the flame rows 10.The gaseous stream emanating between the flame rows 10 of the heater 8heats the powdered substance to be sprayed.

The heater -8 of this invention ensures such a surprisingly high degreeof thermal efficiency that it is possible to spray not only syntheticresin powders, but, with the use of a gas-air mixture, also powderedaluminum.

The gas pressure relationships can also be selected so that the heater 8remains untouched by the flame core and thus the burner is keptcompletely cold during operation and even cools the exiting gaseousstream. Accordingly, it is also possible to avoid any adherence of thepowder, and to ensure the spraying thereof without combustion, withsafety, without the use of any special measures. An additional advantageresides in that the heating of the cold powder flowing out of the coldnozzle 7 begins in the mixing zone of the combustion gas mixture and thepowder jet 9, at a distance of about 0.3-0.4 inch from the front of theheater 8, in the free space or between surfaces serving for heatingpurposes; consequently, strongly adhering powders such as, for example,powdered synthetic resins, do not adhere anywhere nor producedisturbances in operation.

As is shown in FIGS. and 6, air or another nonflammable gas is allowedto flow, in place of fuel gas,

through the heater 8 and the pipe 11 of the device of this invention; asa consequence thereof, incombustible air emanates from these openings inplace of the raw of rflames, which air then prevents the lateralexpansion of the powder jet 9. Upstream of the heater 8, the powder jet9 is sandwiched between energy supply elements 12 in order to transferthermal or electric energy to the powder jet 9.

The heater 8 provided for ensuring the lateral expansion of the powderjet 9 can also be entirely omitted in certain cases, in which case thepowder jet 9 exiting from the nozzle 7 is then immediately supplied withthe required thermal or electric energy by the energy supply elements12.

In view of the foregoing, it will be evident that all of the initiallyrecited objects of the present invention have been achieved.

Although the present invention has been described and illustrated inconnection with preferred embodiments, it is to be understood thatmodifications and variations may be resorted to without departing fromthe spirit of the invention, as those skilled in this art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the present invention as defined by theappended claims.

Having described my invention, I claim:

1. A method for spraying powdered thermoplastic substances onto a solidsubstrate, comprising entraining the powdered substance in a coldgaseous stream, projecting said stream from an outlet opening in theform of a flat cloud against a said substrate, projecting a pair ofgaseous streams from outlet openings that are on opposite sides of andforward of the first-mentioned outlet opening, laterally confining theprojected stream between said pair of gaseous streams, sucking airbetween said pair of gaseous streams from behind and on opposite sidesof said first-mentioned outlet opening, and applying heat to oppositesides of said fiat cloud while confined between said pair of streams.

2. A method as claimed in claim 1, in which said heating is effected byheating said pair of streams.

3. -A method as claimed in claim 1, in which said heating is effected bypassing said projected stream between opposed heating means.

4. A method as claimed in claim 1, in which the thickness of saidprojected stream is from twice the diameter of the powder particles tobe sprayed, up to about inch.

5. A method as claimed in claim 4, in which the breadth of said cloud isnot more than ten inches.

6. A method as claimed in claim 1, and directing each of said pair ofstreams towards said projected stream at an angle of 5 to 20.

7. A method as claimed in claim 1, in which said pair of streams areflames.

8. Apparatus for spraying powdered thermoplastic substances onto a solidsubstrate, comprising a spray nozzle having an elongated flat outletopening, means to feed to said nozzle and out said outlet opening astream of powdered thermoplastic substance in the form of a flat cloudof particles entrained in a relatively cold gas, means to direct streamsof gas from opposite sides through further outlet openings at an acuteangle against opposite sides of the flat cloud of particles emergingfrom said nozzle thereby to confine said flat cloud between saidstreams, said outlet opening of said nozzle being to the rear of saidfurther outlet openings, means to suck air forwardly between saidfurther outlet openings and on opposite sides of said nozzle outletopening, and means to heat said fiat cloud between said streams.

9. Apparatus as claimed in claim '8, said nozzle having a width not morethan 10 inches.

10. Apparatus as claimed in claim 8, said streams being directed againstsaid flat cloud at an angle of 5 to 20.

11. Apparatus as claimed in claim 8, said heating means comprising aburner from which :said streams emerge as flames.

12. Apparatus as claimed in claim 8, said heating means comprising apair of heating elements between which said cloud passes between saidnozzle and said substrate.

13. Apparatus as claimed in claim 8, said feed means including anejector having an inlet opening for powdered substance horizontally tothe side of the ejector thereby to avoid clogging of the ejector withsaid powdered substance.

14. Apparatus as claimed in claim 13, said feed means comprising avibratory conveyor feeding to a hopper which in turn feeds to an ejectorby which said particles are entrained in a gas stream.

References Cited UNITED STATES PATENTS 2,673,121 3/1954 Brennan 239-812,900,138 8/1959 Strate 239-85 X 2,418,533 4/1947 Walker 239-4222,721,147 10/1955 Sullivan 1 17-105.1 2,746,883 5/1956 Powers 117105.2 X3,449,145 6/ 1969 Bloom et a1 117-46 FS FOREIGN PATENTS 658,353 1/ 1965Belgium t. 239-85 533,391 11/1956 Canada 239-79 554,208 6/ 1943 GreatBritain 239-81 1,111,995 7/ 1961 Germany 239-79- 921,924 11/ 1954Germany 239-422 629,417 3 1963 Belgium 2 39-85 ROBERT S. WARD, JR.,Primary Examiner U.S. Cl. X.R.

